Use $root:default as a "default" rule

tools/gn/build_settings.h

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef TOOLS_GN_BUILD_SETTINGS_H_
 #define TOOLS_GN_BUILD_SETTINGS_H_
 
 #include <map>
 #include <memory>
 #include <set>
 #include <utility>
 
 #include "base/callback.h"
 #include "base/files/file_path.h"
 #include "base/macros.h"
 #include "tools/gn/args.h"
+#include "tools/gn/label.h"
 #include "tools/gn/scope.h"
 #include "tools/gn/source_dir.h"
 #include "tools/gn/source_file.h"
 
 class Item;
 
 // Settings for one build, which is one toplevel output directory. There
 // may be multiple Settings objects that refer to this, one for each toolchain.
 class BuildSettings {
  public:
   typedef base::Callback<void(std::unique_ptr<Item>)> ItemDefinedCallback;
   typedef base::Callback<void(const std::string&)> PrintCallback;
 
   BuildSettings();
   BuildSettings(const BuildSettings& other);
   ~BuildSettings();
 
+  // Root target label.
+  const Label& root_target_label() const { return root_target_label_; }
+  void SetRootTargetLabel(const Label& r);
+
   // Absolute path of the source root on the local system. Everything is
   // relative to this. Does not end in a [back]slash.
   const base::FilePath& root_path() const { return root_path_; }
   const std::string& root_path_utf8() const { return root_path_utf8_; }
   void SetRootPath(const base::FilePath& r);
 
   // When nonempty, specifies a parallel directory higherarchy in which to
   // search for buildfiles if they're not found in the root higherarchy. This
   // allows us to keep buildfiles in a separate tree during development.
   const base::FilePath& secondary_source_path() const {
@@ skipping 53 matching lines @@
   // A list of files that can call exec_script(). If the returned pointer is
   // null, exec_script may be called from anywhere.
   const std::set<SourceFile>* exec_script_whitelist() const {
     return exec_script_whitelist_.get();
   }
   void set_exec_script_whitelist(std::unique_ptr<std::set<SourceFile>> list) {
     exec_script_whitelist_ = std::move(list);
   }
 
  private:
+  Label root_target_label_;
   base::FilePath root_path_;
   std::string root_path_utf8_;
   base::FilePath secondary_source_path_;
   base::FilePath python_path_;
 
   SourceFile build_config_file_;
   SourceFile arg_file_template_path_;
   SourceDir build_dir_;
   Args build_args_;
 
   ItemDefinedCallback item_defined_callback_;
   PrintCallback print_callback_;
 
   std::unique_ptr<std::set<SourceFile>> exec_script_whitelist_;
 
   BuildSettings& operator=(const BuildSettings& other);  // Disallow.
 };
 
 #endif  // TOOLS_GN_BUILD_SETTINGS_H_